Take blocks into account into Diviner highlighter

Summary: This is still far from ideal but at least solves the pretty common use of code block with C++-style comments in Remarkup.

Test Plan:
  lang=plain
  lang=remarkup
  a//b//

    a // a
    b // c

  //d
  e//

  //f

  g//

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5094